Digital Subscriber Line (“DSL”) is a developing modem technology that allows existing copper telephone lines to carry high bandwidth information. Familiar twisted-pair telephone lines are then able to carry high speed data communication to and from a customer site in addition to retaining a plain old telephone service (“POTS”) channel for voice communication. At the customer site, a DSL modem receives the downstream signal representing data for a customer from a central office for a telephone company and transmits an upstream signal representing data from the customer to the central office.
One configuration of DSL is generally termed Asymmetric Digital Subscriber Line (“ADSL”). ADSL allows high-speed data transport to the customer site but only medium-speed data transfer from the customer site, whence the asymmetry. ADSL also allows the simultaneous use of the ADSL modem and POTS on the same telephone line. In this configuration, at a point where the twisted-pair telephone line enters the customer site, filters split the POTS channel from the ADSL modem channels. One problem with this type of ADSL, however, is that a technician from the telephone company has to visit the customer site and install the splitter, referred to as “the truck roll.”
To overcome this problem, another configuration of ADSL forgoes the requirement of a splitter at the customer site. This configuration is generally called “splitterless ADSL” and also goes under the name of “G.Lite.” Splitterless ADSL is described in the International Telecommunications Union-Telecommunication Standardization Sector (hereinafter “ITU-T”, formerly known as the CCITT) Recommendation G.992.2, February 1999, which is incorporated herein by reference. ITU-T standards can be found on the World Wide Web at the Universal Resource Locator (“URL”) “www.itu.ch.” Splitterless ADSL modems include those manufactured by 3Com Corporation of Santa Clara, Calif., Lucent Technologies of Murray Hill, N.J., Texas Instruments of Dallas, Tex., and others.
In splitterless ADSL, an ADSL modem directly shares the same twisted-pair telephone line as a POTS telephone without the intervention of a splitter. Sharing the same twisted-pair telephone line, however, may introduce noise into the downstream data transfer. The noise effect is the response of the internal circuitry of the telephone both in the on-hook and off-hook conditions to the upstream ADSL signal. Indeed, any non-linear device sharing the telephone line may introduce noise into the downstream data transfer. Moreover, each telephone or non-linear device has a different response to the upstream ADSL signal: some devices introduce little downstream noise, such as many of the cordless telephones, while others devices introduce a lot of noise, thus severely reducing the rate at which the modem can receive data.
It is desirable, therefore, to reduce the noise that the non-linear device, such as a telephone, introduces when it shares the telephone line with the modem. It is also desirable to achieve noise reduction for a wide range of non-linear devices. This may allow optimal downstream data transfer regardless of what non-linear device shares the telephone line with the ADSL modem.